1. Field of the Invention
The present invention relates generally to a pressure control valve unit. More specifically, the invention relates to a electromagnetic proportioning valve unit which is suitable for use in attitude change suppressive suspension control to be performed in suspension control system, such as active suspension control and so forth.
2. Description of the Background Art
In the recent years, advanced technologies have been developed in automotive suspension systems for achieving both of high level riding comfort and driving stability. In one of such advanced automotive suspension technologies, an active suspension control has been known as one of the most advanced technology in controlling vehicular attitude. Generally, typical construction of active suspension control system includes a hydraulic cylinder disposed between a vehicle body and a suspension member which rotatably supports a road wheel, and defines a working chamber. The working chamber of the hydraulic cylinder is connected to a pressurized fluid source, such as an engine driven fluid pump. A pressure control valve is disposed within a hydraulic circuit connecting the fluid source and the working chamber so as to control the working fluid pressure in the working chamber so as to perform active suspension control for suppressing vehicular attitude change for better riding comfort and higher driving stability. One of typical construction of actively controlled suspension system has been disclosed in the U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987. In addition, Japanese Patent First (unexamined) publication (Tokkai) Showa 58-156784 discloses a pressure control valve applicable for such actively controlled suspension system. The disclosed pressure control valve in the Japanese Publication has a valve housing in which is defined a pressure control chamber disposed therein a main valve spool and a pilot chamber disposed therein a pilot valve spool are housed. The pilot valve is associated with an electromagnetic actuator, such as a solenoid. The pilot valve is operated by the electromagnetic actuator for generating a pilot pressure in the pilot chamber for controlling the main valve spool position for controlling supplying and draining the working fluid pressure in the working chamber.
The valve housing is formed with a main inlet port communicating with a pressure control chamber which is, in turn, communicated with the working chamber of the hydraulic cylinder and a pilot inlet chamber. Also, the valve housing is also formed with a main drain port and a pilot drain port connected to the fluid source for returning the working fluid to the fluid source. The main and pilot inlet ports are commonly connected to a common supply line, and the main and pilot drain ports are connected to a common drain line. Since the main valve spool is placed for blocking the main inlet port when the fluid force in the pilot chamber and the fluid force of the working chamber exerted on the main valve spool. The pilot valve spool blocks the pilot inlet port when the fluid pressure in the working chamber reaches a target pressure.
By connecting the main and pilot drain ports to the fluid source via the common drain line, relatively large amount of working fluid flowing through the drain line from the working chamber when the pressure in the working chamber is to be reduced can affect to the pilot pressure by generating a surge pressure to be introduced into the pilot chamber via the pilot drain port via the drain line. Namely, when such surge pressure in the drain line is introduced into the pilot pressure, pressure balance is destroyed for causing fluctuation of the working fluid pressure in the working chamber. This fluctuation of the pressure in the working chamber can cause generation of the vibration and noise in the hydraulic cylinder.